Docosahexaenoic acid is a polyunsaturated fatty acid which is present specifically in the brain or retina of an animal. It plays a major physiological role in those organs, and also has physiological activities such as an anti-inflammatory effect or an effect of lowering blood cholesterol level. Accordingly, docosahexaenoic acid is a useful substance, the use of which has drawn attention in the fields of medicines and food products. Recently, use of docosahexaenoic acid has also been expanded in the field of functional foods such as health foods and formulas.
Docosahexaenoic acid is contained in fish oil of bluefish. More particularly, about 20% of sardine- or tuna-derived oil is made up of docosahexaenoic acid. A major problem in terms of the use of fish-derived docosahexaenoic acid-containing fat and oil in the field of food products is necessity of considerable operations for removing the smell of fish. Furthermore, since fish-derived docosahexaenoic acid-containing fat and oil contains a variety of polyunsaturated fatty acids such as arachidonic acid or icosapentaenoic acid (EPA), it is likely to be oxidized. Thus, it is difficult to obtain fat and oil having stable quality. Further, when docosahexaenoic acid is used in the field of medicines or the like, it is necessary that docosahexaenoic acid is separated and purified from docosahexaenoic acid-containing fat and oil. However, a variety of polyunsaturated fatty acids having structures similar to that of docosahexaenoic acid are contained, and thus, separation and purification are difficult. In the preparation of formulas, docosahexaenoic acid-containing fat and oil having a low content of icosapentaenoic acid is particularly preferable. However, when the source is fish oil, it is very difficult to efficiently remove icosapentaenoic acid only.
As a source of docosahexaenoic acid other than fish oil, a method for producing docosahexaenoic acid using microorganisms have been attempted. Examples of known microorganisms which produce docosahexaenoic acid-containing fat and oil include Vibrio marinus (bacteria extracted from the deep sea); Vibrio (bacteria separated from the intestine of deep-sea fish); Thraustochytrium aureum which is Mastigomycotina; Japonochytrium sp.; and Cyclotella cryptica, which is a type of microalgae. Production of docosahexaenoic acid-containing fat and oil by culture techniques utilizing these microorganisms have also been studied (JP Patent Publication (Kokai) No. 1-199588 A (1989)). However, according to the conventionally known methods utilizing microorganisms, the production amount of docosahexaenoic acid-containing fat and oil per liter of medium was as small as approximately 100 mg to 700 mg, and the production amount of docosahexaenoic acid was also very small.
JP Patent Publication (Kokai) No. 10-72590 A (1998) and JP Patent No. 2764572 disclose the production of docosahexaenoic acid using the Schizochytrium sp. strain (SR21). In these documents, drawbacks such as small production amounts, necessity of a special medium for culturing, and necessity of new capital investment for equipment for production are also pointed out.
JP Patent Publication (Kohyo) No. 2000-513575 discloses that fat and oil can be collected from cultured microorganisms of the genus Ulkenia that has an ability of producing docosahexaenoic acid- and docosapentaenoic acid-containing fats and oils, but there are same drawbacks as mentioned above.
Rapeseed oil is often used as a starting material for long-chain saturated fatty acid (C22:0 behenic acid) which is necessary for photographic photosensitive materials. A rapeseed breed containing no C22 component has become preferred, and the breed improvement thereof has been in progress. Thus, the supply of a rapeseed breed containing C22 component tends to decrease. In addition, hydrogenated rapeseed oils contain fatty acid having similar numbers of carbon atoms, and thus, laborious purification and cost are of serious problem for the production of highly purified behenic acid. Therefore, it is desired to develop a technique for producing useful lipids, which can stably provide highly purified DHA and behenic acid at a low cost.